It is customary to provide bar codes on dried, slide-like test elements used in analyzers to test for analytes of liquids. The bar-code information provides machine-readable information such as the type of assay for that element, and optionally, calibration data needed to calibrate that particular test element.
The test element body is typically a plastic material, such as polystyrene. This underlying material is not an easy material to print on. Thus, conventional printing techniques have been difficult to use as they do not always provide a sufficiently contrasting color. A preferred format is to use a white test element, hereinafter "slide", with a dark or black ink. Although flexographic printing of a black ink has been used, this process suffers the disadvantages of high cost, the need to print in continuous web format and possible voids in the printing.
Therefore, there has been a need prior to this invention to develop a method of marking the plastic slides by a technique other than printing.
Laser marking has been developed for articles in general, as described, for example, in U.S. Pat. Nos. 4,323,755 and 4,515,867. However, there is a problem in applying this technique to plastic articles in general, and particularly to polystyrene articles. That is, the coating that is to be ablated away to reveal the contrasting color of the native plastic underneath, is not readily ablatable without damaging the plastic with the laser. That is, the contrasting color coating adheres so well to the plastic that the last molecules of the color do not readily ablate without unduly damaging the plastic. This is particularly a problem with certain plastic articles, such as polystyrene slides, that contain the dried reagents used to assay for liquid analytes. That is, these plastic articles, if ablated so as to remove some of the plastic, will tend to flow so as to become warped or distorted.
It is not entirely clear what the mechanism is for the tenacious binding of the contrast color coating to the plastic. It may be solvent attack of the plastic, inherent porosity of the plastic surface, or even a binding that is enhanced by the heat of the ablation process. It appears to occur whether or not the contrast color coating is solvent-based or aqueous-based.
Thus, prior to this invention the need for a non-printing marking technique for use on plastic articles was not met by laser-ablation techniques.
An additional problem arising from marking by laser ablation is the tendency of the laser process to cause redepositing of the contrast coating in other areas. For this reason, laser marking of lot numbers on bottles has in the past used large dark areas and an ablation in the middle of that area, so that the ablation debris falls onto the bordering dark area, rather than a white area. The disadvantage of this approach is that considerable area is wasted in the large dark area that is required.
Thus, a further problem has been to mark any substance with laser ablation, without leaving debris behind in white areas and without requiring large areas of dark borders.